Most common cause of over pressurising is air locks, they can be a real bitch to get all the air out. If there has been a coolant leak and it has been allowed to get too low, then it will need bleeding properly. Common causes of leaks are from the heater core O rings where coolant drips down the centre console next to the drivers left foot or from the throttle body heater. Difficulty bleeding is very regularly down to the bleed nipple on the top of the radiator, the pipe attached to it and the hole in the header tank being blocked so any air trapped in the top of the radiator can't get out. They do go through head gaskets, especially if they have been overheated. It's the standard neglect makes the problem worse. A coolant leak gets ignored so the level drops, the lack of coolant causes an overheat which distorts the heads and causes the gasket to blow. Slipped liners are much rarer than some people make out and the porous block story is, in 99.99% of cases just that, a story but a serious overheat can cause all sorts of damage which may not be immediately apparent. If head gasket symptoms are noticed, check compressions and plugs for any signs of coolant. If you've got one of those little cameras you can stick down a spark plug hole, use that to look for a steam cleaned piston. The coolant passages run at each end of the heads so if coolant is getting into the front or rear cylinders on a bank, it is most likely from the head gasket, if it's either of the middle ones, it points to a weeping liner. Liner problems don't just happen though, they are caused and the cause is almost always that the engine has been seriously overheated. An overheat will always be caused by one of two things, a lack of coolant or a lack of flow. Again we come down to neglect or penny pinching. Using plain water as coolant clogs the coolant system with rusty stuff, that clogs the radiator resulting in poor flow. Water pumps do wear out, fitting a Britpart (although the Br is often substituted with Sh) at under £20 is wasting money. If you are lucky they will start to leak after about 2,000 miles, if you are unlucky they don't leak but the impeller falls off the shaft so everything looks perfect but it's doing nothing. Anything like this will increase the possibility of an overheat which can lead to further problems sometime later.
100% cure is to fit top hat liners (£950 from V8 Developments that did mine) as this puts the fire ring onto the top of the liner rather than on the block with the join between block and liner within the combustion chamber. Not cheap but you end up with an engine that will be good for another 250,000 miles. The problem is that these cars were very expensive when new and while they were owned by the first or second owner, they were properly serviced. Now they can be picked up for under a grand but will almost certainly have been neglected in the meantime.
'96 Saab 900XS, AEB Leo, sold
'93 Range Rover 4.2 LSE, Lovato LovEco, sold
'98 Ex-Police Range Rover 4.0, Singlepoint AEB Leo, my daily motor
'97 Range Rover 4.0SE, eGas multipoint, a project.....
Proud to be a member of the YCHJCYA2PDTHFH club.